Basically, Knight Templar wants Star Trek to feature the kind of diverse personalities that would realistically never make it past the presumably thorough psych evaluations that Starfleet undoubtedly requires all of its cadets/personnel. If someone's motivations for exploring the galaxy are rooted in getting revenge, or fulfilling some fanatical quest, or anything else so psychotic as what he's suggested as possible motivations, one would hope that they'd be culled from the program prior to graduating from the Academy, and would never, ever make it so far as a shipboard posting.
Lets suppose the USS Discovery, a Galaxy Class starship was conducting an away mission, and the captain was the last to beam back aboard his ship, unfortunately for him, he was caught in an ion storm when this happened and ended up in the Mirror Universe as a rogue rebel, the rogue rebel, who was conducting sabotage operations against a Klingon outpost suddenly finds himself in a starfleet uniform with the rank of captain, he's never before commanded such a large starship, but his situation is much improved from what it was before. Fortunately he picks up the clues rather quickly about what he is supposed to be and is a good actor, of course he is an impostor, but he keeps up the charade for as long as possible. How long do you think he could keep it up?